An electrochromic (EC) phenomenon is a phenomenon in which a material is colored or decolored through changes in its light absorption region induced by a reversible electrochemical reaction (oxidation reaction or reduction reaction) caused at the time of application of a voltage. An electrochemically coloring/decoloring element utilizing the EC phenomenon is referred to as “electrochromic element (EC element),” and is expected to find applications as a light control element configured to change an optical transmittance. It has been known that an organic EC element, in which a low-molecular weight organic material is colored/decolored in a solution state, has advantages of a sufficient contrast ratio in a colored state, a high transmittance in a decolored state, and the like. In addition, it has been known that the organic EC element has an advantage in that its color state can be arbitrarily controlled by mixing a plurality of materials having different absorption wavelengths. The use of such EC element in an optical filter requires not only arbitrary control (gradation control) of the optical transmittance but also the fact that the wavelength selectivity (absorption spectrum) of light absorption does not largely change.
A voltage modulation method involving changing the magnitude of a drive voltage or a pulse width modulation method involving changing the pulse width (duty ratio) of a voltage application period has been known for controlling the gradation of the EC element. In PTL 1, gradation control is performed in an EC element using a low-molecular weight organic EC material by the voltage modulation method. In the voltage modulation method, when an oxidation-reduction potential difference between anodic materials or between cathodic materials is large, a difference occurs in a reaction amount ratio between the materials owing to a difference in drive voltage to preclude the retention of the shape of an absorption spectrum. In PTL 1, the oxidation-reduction potential difference between the anodic materials or between the cathodic materials is set to 60 mV or less for solving the problem. The change of the absorption spectrum due to the difference in drive voltage is suppressed by uniformizing their oxidation-reduction potentials.
However, when the EC element is driven by the voltage modulation method, an increase in applied voltage involves, for example, the following problems. An electrical load is liable to be applied to an EC material to accelerate its deterioration, and the influences of impurities, such as water and oxygen, on electrical characteristics are liable to be apparent.
Accordingly, the pulse width modulation method is preferred as a driving method. In the pulse width modulation method, a voltage application period, i.e., the period for which an electrochemical reaction is controlled occupying one cycle of a pulse is adjusted by making an applied voltage constant. An excessive electrical load on the EC material and the influences of the impurities on the electrical characteristics observed in the voltage modulation method are suppressed because the driving is performed under a constant voltage. Further, a difference seldom occurs in a reaction amount ratio between materials, and hence even in the case of anodic materials or cathodic materials having different oxidation-reduction potentials, an absorption spectrum can be easily retained against gradation control. As described above, in the EC element, the use of the pulse width modulation method under a constant voltage enables the performance of gradation control in a state in which the absorption spectrum is retained.
However, an EC element having a plurality of anodic materials or a plurality of cathodic materials involves a problem in that its absorption spectrum changes with a driving environment temperature. That is, in the case where the anodic materials or the cathodic materials differ from each other in temperature dependence of an electrochemical reaction, even when the element is driven under a constant voltage, a difference occurs in a reaction amount ratio between the anodic materials or the cathodic materials as the driving environment temperature changes. As a result, the shape of the absorption spectrum cannot be retained.